This invention relates generally to an implantable cardiac stimulation system capable of automatically performing threshold tests, and more specifically, to a method for verifying that the result of a given threshold test is not due to fusion activity.
In the normal human heart, the sinus node, generally located near the junction of the superior vena cava and the right atrium, constitutes the primary natural pacemaker initiating rhythmic electrical excitation of the heart chambers. The cardiac impulse arising from the sinus node is transmitted to the two atrial chambers, causing a depolarization known as a P-wave and the resulting atrial chamber contractions. The excitation pulse is further transmitted to and through the ventricles via the atrioventricular (A-V) node and a ventricular conduction system causing a depolarization known as an R-wave and the resulting ventricular chamber contractions. Disruption of this natural pacemaking and conduction system as a result of aging or disease can be successfully treated by artificial cardiac pacing using implantable cardiac pacing devices, including pacemakers and implantable defibrillators, which deliver rhythmic electrical pulses or other anti-arrhythmia therapies to the heart at a desired energy and rate. One or more heart chambers may be electrically stimulated depending on the location and severity of the conduction disorder.
Modern pacemakers and implantable defibrillators possess numerous operating parameters, such as pacing pulse energy, base pacing rate, sensing threshold, pacing mode, etc., that must be programmed by the physician to satisfy individual patient need. In practice, this programming process can be time consuming and complicated. A common goal of pacemaker manufacturers, therefore, is to fully automate pacemaker function in order to minimize the complexity of programming operations and to maximize the safety and effectiveness of the cardiac pacing device.
One function of the cardiac stimulating device is therefore to deliver a stimulation pulse of sufficient energy to depolarize the cardiac tissue causing a contraction, a condition commonly known as xe2x80x9ccapture.xe2x80x9d One approach to ensure capture is to deliver a fixed high-energy pacing pulse. While this approach, used in early pacemakers, is straightforward, it quickly depletes battery energy and can result in patient discomfort due to extraneous stimulation of surrounding skeletal muscle tissue.
Preferably, stimulation pulses are delivered at, or slightly higher than the capture xe2x80x9cthreshold.xe2x80x9d Capture threshold is defined as the lowest stimulation energy at which capture occurs. By stimulating the heart chambers at, or just above threshold, comfortable and effective cardiac pacing is provided without unnecessary depletion of battery energy. Capture threshold, however, is extremely variable from patient-to-patient due to variations in electrode systems used, electrode positioning, physiological and anatomical variations of the heart itself, and so on. Therefore, at the time of device implant, the capture threshold is determined by the physician who observes an ECG recording while pulse energy is decreased starting from a high level, either by decrementing the pulse amplitude or the pulse width, until capture disappears.
The pacing pulse energy is then programmed to a setting equal to the lowest pulse energy at which capture still occurred (threshold) plus some safety margin to allow for small fluctuations in threshold. Selection of this safety margin, however, can be arbitrary. Too low of a safety margin may result in loss of capture, with a potentially fatal result for the patient. Too high of a safety margin will lead to premature battery depletion and potential patient discomfort.
Furthermore, pacing threshold will vary over time within a patient, for example, as fibrotic encapsulation of the electrode occurs during the first few weeks after surgery. Fluctuations may even occur over the course of a day or with changes in medical therapy or disease state. Hence, techniques for monitoring the cardiac activity following delivery of a stimulation pulse have been incorporated in modem pacemakers in order to verify that capture has indeed occurred. If a loss of capture is detected by such xe2x80x9ccapture-verificationxe2x80x9d algorithms, a threshold test is performed by the cardiac stimulating device in order to re-determine the capture threshold and automatically adjust the stimulation pulse energy. This approach, called xe2x80x9cautocapturexe2x80x9d, improves the patient""s comfort, reduces the necessity of unscheduled visits to the medical practitioner, and greatly increases the device battery life by conserving the energy used to generate stimulation pulses.
One technique for determining whether capture has occurred is monitoring the myocardial electrogram (EGM) received on the cardiac pacing and sensing electrodes. Heart activity is monitored by the cardiac stimulation device by keeping track of the stimulation pulses delivered to the heart and examining the EGM signals for evidence of depolarization or contraction of muscle tissue (myocardial tissue) of the heart immediately following stimulation delivery. Through sampling and signal processing algorithms, the presence of an xe2x80x9cevoked responsexe2x80x9d, either an intracardiac P-wave or R-wave, following a pacing pulse can be determined. The xe2x80x9cevoked responsexe2x80x9d is the depolarization of the heart tissue in response to a pacing pulse, in contrast to the xe2x80x9cintrinsic responsexe2x80x9d which is the depolarization of the heart tissue in response to the heart""s natural pacemaking function. Detection of an evoked response indicates capture was achieved.
However, it is for several reasons very difficult to detect a true evoked response. One problem commonly encountered during capture verification is xe2x80x9cfusion.xe2x80x9d Fusion occurs when a pacing pulse is delivered such that the evoked response occurs coincidentally with an intrinsic depolarization. The evoked signal may be absent or altered preventing correct capture detection by the pacemaker""s capture detection algorithm. A loss of capture may be indicated when capture is in fact present, an undesirable situation that will cause the pacemaker to unnecessarily deliver a high-energy back-up pacing pulse, and to invoke the threshold testing function in a chamber of the heart.
An even more adverse affect of fusion is when fusion occurs during a threshold test causing an erroneously high threshold result. Automatic adjustment of the pacing pulse energy to this higher output will waste battery energy until the next threshold test is initiated. In extreme situations, fusion could persist throughout a threshold test, driving the pacing energy output to a maximum level.
A common practice in minimizing the likelihood of fusion during a threshold test is temporarily changing stimulation parameters, e.g. base pacing rate and/or the atrio-ventricular delay. Increasing the pacing rate a given level above the sensed rate during the threshold test decreases the likelihood that the pacing pulse will be delivered coincidentally with an intrinsic response. During dual chamber pacing, the atrio-ventricular (AV) delay can be shortened such that pacing in the ventricle will safely occur earlier than the anticipated intrinsic ventricular response. However, while changing stimulation parameters may lessen the chance of fusion occurring during a threshold test, it does not guarantee that fusion will not occur.
One method of performing a threshold test is to start at a pacing pulse energy at which capture is already occurring, i.e. supra-threshold, and progressively decrement pulse energy until capture is lost. The lowest pulse energy at which capture is maintained is deemed the threshold value. Another method of performing a threshold test is to start at a sub-threshold pulse energy and progressively increment pulse energy until capture is achieved. Again, the lowest pulse energy at which capture is maintained is considered the threshold value.
These two methods, however, will consistently arrive at different threshold results, even in the same patient under the same set of circumstances, due to a phenomenon known as the xe2x80x9cWedensky Effect.xe2x80x9d The threshold result from the first technique of progressively decrementing pulse energy from a supra-threshold level will provide a lower threshold result than the second technique of starting from a sub-threshold level and progressively incrementing pulse energy. This phenomenon can best be described as a facilitating effect of electrically excitable tissue. Once excitable tissue has already been depolarized by an electrical impulse, less energy is required to continue exciting the tissue then the energy that was required to initially depolarize the tissue from an unexcited state. This effect is also referred to as xe2x80x9cthreshold hysteresis.xe2x80x9d
Clinical studies of patients having implanted cardiac stimulating devices have found that the difference between the two threshold results, hereafter referred to as the Wedensky Effect value, is constant regardless of the actual threshold levels, the duration of device implant, disease state, or intrinsic cardiac activity (reference is made to xe2x80x9cPacing Threshold Interval with Decreasing and Increasing Output,xe2x80x9d Sylven et al, PACE, 5:646-9, 1982).
It would thus be desirable to provide a system and method for reliably and accurately determining capture threshold. It would further be desirable to determine when a threshold measurement is likely to have been affected by fusion. Furthermore, when a threshold result is suspected of being erroneous due to fusion, it is desirable to re-determine threshold by performing a new threshold test under conditions that minimize the likelihood of fusion. By reliably and accurately determining pacing threshold, battery energy is conserved, improving overall device performance.
The present invention addresses these and other concerns by providing an implantable cardiac stimulation device with automatic threshold testing capabilities. One aspect of the present invention is to provide a method for measuring a first threshold and a second threshold. The first threshold is determined by decreasing stimulation energy from an initially high, supra-threshold value until loss of capture is detected. The pulse energy at which capture is lost is the first threshold value. The second threshold value is determined by increasing the stimulation energy from an initially low, sub-threshold value until capture is detected. The stimulation energy at which capture is regained is the second threshold value.
Another aspect of the present invention is to provide a method for determining if the threshold test results are reliable, by comparing the difference between the first and second threshold values to an expected difference, known as the Wedensky Effect, for a particular patient. Therefore, one feature of the present invention is the automatic determination of the expected Wedensky Effect for the particular patient by calculating a mean Wedensky Effect and its standard deviation based on all previous threshold tests performed on that patient. A deviation from the Wedensky Effect value known for a given patient indicates a discrepancy in the threshold test result. Such a deviation is used to indicate when a threshold test is suspected of being erroneous, for example, due to fusion. For example, if the difference between the first threshold value and the second threshold value is a given amount greater than the mean Wedensky Effect value, the threshold test is suspected of being erroneous due to fusion occurring during the threshold test.
A further aspect of the present invention is a method for redetermining threshold when a threshold test result is suspected of being erroneous due to fusion. During a second threshold test, stimulation parameters are adjusted so that the likelihood of fusion is minimized. Once a threshold test result is evaluated and found to be consistent with the expected Wedensky Effect value, the stimulation energy is automatically adjusted to a level safely above the threshold value.
Thus, the present invention teaches a cardiac stimulating device and electrode system capable of delivering stimulating pulses to a given heart chamber and detecting when capture occurs. The present invention further includes a method for automatic threshold testing. The associated method of the present invention determines whether fusion may have interfered with obtaining accurate threshold test results, and automatically adjusts the stimulation energy based on the threshold test results once they have been verified as being reliable. This automatic adjustment of stimulation energy maintains the minimum energy necessary for effective stimulation therapy delivery thus improving battery longevity.